The present invention relates to electrical connectors for mounting to printed circuit boards, and more particularly to an improved connector having terminals with improved solder tails.
Devices such as computers using printed circuit boards are exhibiting increasing circuit densities and are operating at increasing frequencies. For example, the speeds of high frequency digital signals traveling between a computer motherboard and densely populated memory module printed circuit cards on an associated circuit board are becoming higher.
These trends create problems for electrical connectors such as edge card connectors that are used to removably mount a circuit card on a circuit board. With increasing circuit density, the electrical connectors and the electrical terminals they include are smaller and closer together. The terminals must nevertheless be sufficiently flexible and strong to provide reliable contact with a circuit card inserted into the connector. In addition, it is desirable to keep small the impedance of the circuit paths provided by the electrical terminals of the edge card connector. Meanwhile, inductance must be kept to a minimum, capacitance must be carefully controlled, and crosstalk between different signals must be minimized.
Yet another problem which may arise with increased circuit density is the undesirable bridging of solder from one terminal tail and corresponding through hole to another. Surface tension shapes molten solder into generally circular fillets around where the terminal tail protrudes from a through hole in a circuit board. Where through holes and corresponding tails are disposed particularly close together, the solder fillets formed about those through holes may overlap, thereby providing an undesirable short circuit between the terminal tails. Therefore, it is also desirable to prevent solder bridging to avoid unwanted short circuits and the appearance of inferior quality in the connector-mounted circuit board.
The various and conflicting goals discussed above have led to many approaches for connector and terminal design with varying degrees of success. U.S. Pat. No. 5,161,987, for example, discloses an electrical connector having a ground bus with a plurality of solder tails. A row of signal contacts is located on each side of the ground bus.
U.S. Pat. No. 5,162,002, meanwhile, discloses a card edge connector with spatially overlapped terminals having relatively shorter and relatively longer contact elements. This connector has important advantages such as reducing the peak card insertion force, but has electrical characteristics that are not optimized for higher speed digital signals.
U.S. Pat. No. 5,192,220 discloses a dual readout socket wherein crosstalk is reduced by increasing the space between connectors. This approach defeats the goal of increased circuit density.
U.S. Pat. No. 5,259,768 discloses an electrical connector having ground terminals with significantly larger surface areas than the signal terminals. The ground and signal terminals alternate, and the shadowing effect of the ground terminals reduces crosstalk. The ground terminals have both solder tails and grounding feet to reduce impedance generally, while non-functional stubs are sized to provide a specifically desired impedance.
U.S. Pat. No. 5,259,793 discloses an edge connector with terminals arranged in an alternating array along the circuit card insertion slot. Circuit density is diminished because of the alternating array.
U.S. Pat. No. 5,309,630 discloses an electrical connector wherein a desired impedance is obtained by selecting terminals having anchoring portions sized to correspond to the desired impedance. Signal and ground terminals may alternate, and at least the ground terminals are provided with two feet to reduce impedance. U.S. Pat. No. 5,580,257 discloses a connector in which enlarged ground terminals are adjacent to pairs of signal terminals to reduce crosstalk. Although this arrangement has advantages, three different terminal shapes are required, and the operation of assembling terminals into the connector housing is complex.
U.S. Pat. No. 5,654,878 discloses a terminal tail having a reduced-width portion wherein parallel edges provide dual alignment positioning allowance on opposite sides of the tip of the solder tail for facilitating insertion thereof into a through hole.
U.S. Pat. No. 5,409,399 discloses solder tails having curved sections for providing transverse offset relative to the centerlines of the solder tails.
Despite these and many other attempts, there remains a long-standing need for a card edge connector that can be made at reasonable cost, is robust and reliable, has high circuit density, performs well in high speed digital circuits, and avoids density related problems such as solder bridging.